The invention relates to a classical side condenser method for discharging components that are less volatile than oxygen from an air separation plant that contains a main heat exchanger, a side condenser and a distillation column system for nitrogen-oxygen separation, wherein the side condenser is constructed as a condenser-evaporator and is arranged in a vessel and the distillation column system for nitrogen-oxygen separation has at least one high-pressure column and a low-pressure column and wherein, in the method a first feed air stream is cooled in the main heat exchanger and is introduced into the high-pressure column, a second feed air stream is cooled in the main heat exchanger and is at least in part liquefied in the side condenser, a first oxygen fraction is withdrawn in the liquid state from the low-pressure column and introduced into the vessel of the side condenser, the sump liquid of the vessel of the side condenser is at least in part vaporized, a second oxygen fraction is withdrawn in the gaseous state from the head of the vessel of the side condenser, the second oxygen fraction is warmed in the main heat exchanger, a third oxygen fraction is taken off at least intermittently as a purge stream from the bottom of the vessel in the liquid state and removed from the distillation column system for nitrogen-oxygen separation and the purge stream is subjected to at least one of the following treatments: discharge into the surroundings, withdrawal as end product, characterized in that: the vessel, above the side condenser has a mass transfer section which corresponds to more than one theoretical plate and fewer than 10 theoretical plates and the oxygen content of the third oxygen fraction is less than the oxygen purity of the first oxygen fraction.
The side condenser is constructed as a condenser-evaporator.
“Condenser-evaporator” denotes a heat exchanger in which a first condensing fluid stream enters into indirect heat exchange with a second vaporizing fluid stream. Each condenser-evaporator has a liquefaction space and a vaporization space which consist of liquefaction passages and vaporization passages, respectively. In the liquefaction space, the condensation (liquefaction) of the first fluid stream is carried out, and in the vaporization space the vaporization of the second fluid stream is carried out. Vaporization space and liquefaction space are formed by groups of passages which are in a heat-exchange relationship with one another.
The vaporization space of a condenser-evaporator can be constructed as a bath evaporator, falling-film evaporator or forced-flow evaporator. A side condenser is generally constructed as a bath evaporator (thermosiphon evaporator) which is heated with a part of the feed air.
In the liquid oxygen product of the low-pressure column (the “first oxygen fraction”) components of the air that are less volatile than oxygen also accumulate. These remain behind in the liquid during the vaporization in the side condenser. In addition to the vaporized product (the “second oxygen fraction”), therefore, a purge stream (the “third oxygen fraction”) must also be removed, in order that the less volatile components do not become enriched. This purging can be performed continuously or discontinuously, and is, for example, 0.2 mol % of the feed air amount.
Methods of the type described in the outset and corresponding devices are known from DE 4327311 A1 or WO 2010017968 A2 (=US 20110214453 A1).
“Remove from the distillation column system for nitrogen-oxygen separation” means here that the corresponding purge stream is not introduced into one of the separation columns of the distillation column system for nitrogen-oxygen separation. The purge stream in the case of the invention, is instead either discharged, that is to say let out into the atmosphere (optionally after external vaporization) or withdrawn as end product, by taking it off, for example as liquid product (as in DE 4327311 A1) or feeding it to another liquid product stream (as in WO 2010017968 A2=US 20110214453 A1) or to a gas product stream.
The object of the invention is to increase the yield of gaseous oxygen product in the form of the second oxygen fraction.
In the case of yield increase, a person skilled in the art first considers increasing the separation action of high-pressure column and/or low-pressure column, by modifying parameters such as pressure, reflux ratio and/or number of theoretical plates. The use of a third column for nitrogen-oxygen separation is also possible.